The present invention relates to a color reproduction system, and relates, more particularly, to a color reproduction system for obtaining an input color image signal for displaying desired colors in a color image display unit capable of displaying a color image of at least four primary colors.
There have been widely used various display units such as CRT displays, liquid crystal displays, liquid crystal projectors, etc. as a means for reproducing digital color images that have been input by scanners, digital cameras, and the like. Usually, these display units reproduce various colors based on an additive color mixture of three primary colors, R, G, and B. In this case, a range of display colors that a display can reproduce, that is, a color reproduction area, is limited to an area that can be displayed as a sum of color vectors of three primary colors in a three-dimensional color space. For example, a CRT display for reproducing colors by three kinds of phosphors R, G and B has a hexahedron as a color reproduction area that has the following vertexes: (0, 0, 0), (Xr, Yr, Zr), (Xg, Yg, Zg), (Xb, Yb, Zb), (Xr+Xg, Yr+Yg, Zr+Zg), (Xg+Xb, Yg+Yb, Zg+Zb), (Xb+Xr, Yb+Yr, Zb+Zr), and (Xr+Xg+Xb, Yr+Yg+Yb, Zr+Zg+Zb). In this case, (Xr, Yr, Zr), (Xg, Yg, Zg) and (Xb, Yb, Zb) represent color displays X, Y and Z respectively of the CIE1931 color display system (XYZ color display system) at the time of a maximum light emission of the R, G and B phosphors.
FIG. 11 is a schematic diagram of a color reproduction area in the XYZ space of the three-primary color display. FIG. 12 is an xy-chromaticity diagram of this schematic diagram. As shown in FIG. 12, the area inside a triangle encircled by chromaticity values of the primary colors is a color reproduction area. In the display that is based on the additive color mixture of three primary colors as the principle of color reproduction, a relationship between the R, G and B signal colors that are input to the display and X, Y and Z values of colors that are displayed on the display is determined uniquely. When the light emission spectrum of each primary color is independent of an output of other primary color, and also when a relative spectrum distribution does not depend on the light emission intensity (that is, when the chromaticity value does not change), X, Y and Z of display colors corresponding to the input R, G and B values are given by the following expression.                               (                                                    X                                                                    Y                                                                    Z                                              )                =                              (                                                                                X                    ⁢                                                                                   ⁢                    r                                                                                        X                    ⁢                                                                                   ⁢                    g                                                                                        X                    ⁢                                                                                   ⁢                    b                                                                                                                    Y                    ⁢                                                                                   ⁢                    r                                                                                        Y                    ⁢                                                                                   ⁢                    g                                                                                        Y                    ⁢                                                                                   ⁢                    b                                                                                                                    Z                    ⁢                                                                                   ⁢                    r                                                                                        Z                    ⁢                                                                                   ⁢                    g                                                                                        Z                    ⁢                                                                                   ⁢                    b                                                                        )                    ⁢                      (                                                                                R                    ′                                                                                                                    G                    ′                                                                                                                    B                    ′                                                                        )                                              (        1        )             R′=γr(R)G′=γg(G)B′=γb(B)In this case, Xr, Xg and Xb are X values at the time of a maximum light emission of R, G and B. Similarly, Yr, Yg and Yb are Y values when the light emissions of R, G and B are maximum, and Zr, Zg and Zb are Z values when the light emissions of R, G and B are maximum. Further, γr, γg and γb are functions that show a relationship between an input signal value and an output luminance of R, G and B respectively. R′, G′ and B′ are assumed to be normalized to become 1 at the time of a maximum light emission of R, G and B respectively. From an inverse relationship of the above, input R, G and B values for displaying desired X, Y and Z are obtained from the following expression. R=γr−1(R′)G=γg−1(G′)B=γb−1(B′)                              (                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              )                =                                            (                                                                                          X                      ⁢                                                                                           ⁢                      r                                                                                                  X                      ⁢                                                                                           ⁢                      g                                                                                                  X                      ⁢                                                                                           ⁢                      b                                                                                                                                  Y                      ⁢                                                                                           ⁢                      r                                                                                                  Y                      ⁢                                                                                           ⁢                      g                                                                                                  Y                      ⁢                                                                                           ⁢                      b                                                                                                                                  Z                      ⁢                                                                                           ⁢                      r                                                                                                  Z                      ⁢                                                                                           ⁢                      g                                                                                                  Z                      ⁢                                                                                           ⁢                      b                                                                                  )                                      -              1                                ⁢                      (                                                            X                                                                              Y                                                                              Z                                                      )                                              (        2        )            
In the Expression (2), −1 shows an inverse function (an inverse matrix in the case of a matrix). As explained above, it is easy to model a relationship between X, Y, Z and the R, G, B values in the case of the three-primary color display. There has been generally used a conversion method that is based on a matrix conversion and a gradation correction as disclosed in “The Theory of Color Image Reproduction (in Japanese)”, Joji Tajima, Maruzen Co., Ltd. When X, Y and Z are in the area outside the color reproduction area of the three-primary color display, any one of the R′, G′ and B′ values obtained in the Expression (2) becomes negative or larger than 1.
According to the display unit that is based on the additive color mixture of primary colors as the principle of color reproduction, the area encircled by the chromaticity values of the primary colors becomes the color reproduction area as described above. In order to expand the color reproduction area, it is considered suitable to increase the chroma of each primary color or to increase the number of primary colors. There has also been an attempt to realize a color reproduction area larger than that of the conventional three-primary color display, by using four primary colors as disclosed in the NHK technical material publication, NHK Broadcasting Technical Research, Tokyo, 1995. FIG. 13 and FIG. 14 show a color reproduction area in the XYZ space and a color reproduction area on an xy-chromaticity diagram of a four-primary color display respectively.
In the case of a multi-primary color display using N primary colors of four or more, X, Y and Z of colors displayed for signal values are obtained by the following expression as an expansion of the Expression (1).                               (                                                    X                                                                    Y                                                                    Z                                              )                =                              (                                                                                X                    ⁢                                                                                   ⁢                    c1                                                                                        X                    ⁢                                                                                   ⁢                    c2                                                                                        X                    ⁢                                                                                   ⁢                    c3                                                                    ⋯                                                                      X                    ⁢                                                                                   ⁢                    c                    ⁢                                                                                   ⁢                    N                                                                                                                    Y                    ⁢                                                                                   ⁢                    c1                                                                                        Y                    ⁢                                                                                   ⁢                    c2                                                                                        Y                    ⁢                                                                                   ⁢                    c3                                                                    ⋯                                                                      Y                    ⁢                                                                                   ⁢                    c                    ⁢                                                                                   ⁢                    N                                                                                                                    Z                    ⁢                                                                                   ⁢                    c1                                                                                        Z                    ⁢                                                                                   ⁢                    c2                                                                                        Z                    ⁢                                                                                   ⁢                    c3                                                                    ⋯                                                                      Z                    ⁢                                                                                   ⁢                    c                    ⁢                                                                                   ⁢                    N                                                                        )                    ⁢                      (                                                                                C1                    ′                                                                                                                    C2                    ′                                                                                                                    C3                    ′                                                                                                ⋮                                                                                                  C                    ⁢                                                                                   ⁢                                          N                      ′                                                                                            )                                              (        3        )             C1′=γ1(C1)C2′=γ2(C2)C3′=γ3(C3)CN′=γN(CN)
A conversion from X, Y and Z into signal values given as an inverse relationship of the Expression (3) is not determined uniquely except the surface of a color reproduction area of the multi-primary color display. Therefore, it is necessary to carry out a unique conversion of X, Y and Z into signal values based on some conditions. One example of a color conversion method in a multi-primary color display is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-261332. According to a first example proposed in this publication, colors are reproduced based on a linear sum of three primary colors selected according to the chromaticity values of input colors. As a second example, there is disclosed a linear conversion using all the multi primary colors.
According to the color conversion method of the first example disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-261332, it is possible to carry out an accurate color reproduction in a range in which colors can be reproduced based on selected three primary colors. However, it is not possible to reproduce all the input colors as this method does not basically take into account a color reproduction area in a luminance direction of the multi-primary color display. Further, according to the color conversion method of the second example disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-261332, the method does not guarantee the obtaining of a solution in which all the primary-color signals are positive. Therefore, in some cases, reproducible input colors cannot be reproduced accurately.
It is desirable that a color conversion method makes it possible to carry out a colorimetrically accurate color reproduction. In other words, it is important that when input tristimulus values have been converted into color image signals and then the signals have been input to a display unit, the display unit can accurately display the input tristimulus values. It is desirable that the input tristimulus values and color image signal values change continuously. It is further desirable that these conditions are satisfied in all the color reproduction area of the display.